High resistivity single crystal silicon ingot and wafer having improved mechanical strength

ABSTRACT

A method for preparing a single crystal silicon ingot and a wafer sliced therefrom are provided. The ingots and wafers comprise nitrogen at a concentration of at least about 1×1014 atoms/cm3 and/or germanium at a concentration of at least about 1×1019 atoms/cm3, interstitial oxygen at a concentration of less than about 6 ppma, and a resistivity of at least about 1000 ohm cm.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. application Ser. No.16/303,195, which was filed Nov. 20, 2018, the disclosure of which isincorporated by reference as if set forth in its entirety. U.S.application Ser. No. 16/303,195 is a national stage application ofInternational Application No. PCT/US2017/036061, filed Jun. 6, 2017, thedisclosure of which is incorporated by reference as if set forth in itsentirety. International Application No. PCT/US2017/036061 claims thebenefit of U.S. provisional application Ser. No. 62/347,143, filed onJun. 8, 2016, the disclosure of which is incorporated by reference as ifset forth in its entirety. International Application No.PCT/US2017/036061 claims the benefit of U.S. provisional applicationSer. No. 62/347,145, filed on Jun. 8, 2016, the disclosure of which isincorporated by reference as if set forth in its entirety.

FIELD OF THE INVENTION

This disclosure generally relates to the production of single crystalsilicon ingots and wafers, the ingots and wafers having low oxygenconcentration, high resistivity, and improved mechanical strength.

BACKGROUND OF THE INVENTION

Single crystal silicon is the starting material in many processes forfabricating semiconductor electronic components and solar materials. Forexample, semiconductor wafers produced from silicon ingots are commonlyused in the production of integrated circuit chips. In the solarindustry, single crystal silicon may be used instead of multicrystallinesilicon due to the absence of grain boundaries and dislocations. Singlecrystal silicon ingots are machined into a desired shape, such as asilicon wafer, from which the semiconductor or solar wafers can beproduced

Existing methods to produce high-purity single crystal silicon ingotinclude a float zone method and a magnetic field applied Czochralski(MCZ) process. The float zone method includes melting a narrow region ofa rod of ultrapure polycrystalline silicon and slowly translating themolten region along the rod to produce a single crystal silicon ingot ofhigh purity. The MCZ process produces single crystal silicon ingots bymelting polycrystalline silicon in a crucible, dipping a seed crystalinto the molten silicon, and withdrawing the seed crystal in a mannersufficient to achieve the diameter desired for the ingot. A horizontaland/or vertical magnetic field may be applied to the molten silicon toinhibit the incorporation of impurities, such as oxygen, into thegrowing single crystal silicon ingot. Although float zone silicon ingotstypically contain relatively low concentrations of impurities, such asoxygen, the diameter of ingots grown using the float zone method aretypically no larger than about 200 mm due to limitations imposed bysurface tension. MCZ silicon ingots may be produced at higher ingotdiameters compared to float zone ingots, but MCZ silicon ingotstypically contain higher concentrations of impurities.

During the process of producing single crystal silicon ingots using theMCZ method, oxygen is introduced into silicon crystal ingots through amelt-solid or melt crystal interface. The oxygen may cause variousdefects in wafers produced from the ingots, reducing the yield ofsemiconductor devices fabricated using the ingots. For example,insulated-gate bipolar transistors (IGBTs), high quality radio-frequency(RF), high resistivity silicon on insulator (HR-SOI), charge trap layerSOI (CTL-SOI), and substrate for GaN epitaxial applications typicallyrequire a low oxygen concentration (Oi) in order to achieve highresistivity.

At least some known semiconductor devices are fabricated using floatzone silicon materials to achieve a low Oi and high resistivity.However, float zone materials are relatively expensive and are limitedto use in producing ingots having a diameter less than approximately 200mm. Accordingly, float zone silicon materials are expensive and unableto produce higher diameter silicon crystal ingots with a relatively lowoxygen concentration.

High quality radio-frequency (RF) devices built on high resistivitysilicon on insulator (HR-SOI) require very high resistivity for goodsecond order harmonic performance (HD2). To maintain the highresistivity of the wafer during device fabrication and packaging, a verylow Oi is required in order to minimize the thermal donor impact of Oiand to avoid formation of PN junctions. However, the mechanical strengthof the low Oi wafer is severely degraded, and these wafers are prone toslip during high temperature process steps in SOI line, EPI reactor, anddevice fabrication steps. This causes a high yield loss both for SOIwafer manufacturers as well device manufacturer.

This Background section is intended to introduce the reader to variousaspects of art that may be related to various aspects of the presentdisclosure, which are described and/or claimed below. This discussion isbelieved to be helpful in providing the reader with backgroundinformation to facilitate a better understanding of the various aspectsof the present disclosure. Accordingly, it should be understood thatthese statements are to be read in this light, and not as admissions ofprior art.

BRIEF SUMMARY OF THE INVENTION

In one embodiments, the present invention is directed to a singlecrystal silicon wafer comprising: two major, parallel surfaces, one ofwhich is a front surface of the single crystal silicon wafer and theother of which is a back surface of the single crystal silicon wafer, acircumferential edge joining the front and back surfaces of the singlecrystal silicon wafer, a bulk region between the front and backsurfaces, and a central plane of the single crystal silicon waferbetween the front and back surfaces of the single crystal silicon wafer,wherein by the bulk region comprises an impurity comprising nitrogen ata concentration of at least about 1×10¹⁴ atoms/cm³, germanium at aconcentration of at least about 1×10¹⁹ atoms/cm³, or a combinationnitrogen at a concentration of at least about 1×10¹⁴ atoms/cm³ andgermanium at a concentration of at least about 1×10¹⁹ atoms/cm³, andinterstitial oxygen at a concentration of less than about 6 ppma (NewASTM: ASTM F 121, 1980-1983; DIN 50438/1, 1978), and further wherein themain body of the single crystal silicon ingot has a resistivity of atleast about 1000 ohm cm.

In one embodiment, the present invention is further directed to a methodof growing a single crystal silicon ingot. The method comprisespreparing a silicon melt, wherein the silicon melt is prepared bymelting polycrystalline silicon in a quartz lined crucible and adding asource of an impurity to the quartz lined crucible, the impuritycomprising germanium, nitrogen, or a combination of germanium andnitrogen; and pulling the single crystal silicon ingot from the siliconmelt, the single crystal silicon ingot comprising a central axis, acrown, an end opposite the crown, and a main body between the crown andthe opposite end, the main body having a lateral surface and a radius,R, extending from the central axis to the lateral surface, wherein themain body of the single crystal silicon ingot comprises nitrogen at aconcentration of at least about 1×10¹⁴ atoms/cm³, germanium at aconcentration of at least about 1×10¹⁹ atoms/cm³, or a combination ofnitrogen at a concentration of at least about 1×10¹⁴ atoms/cm³ andgermanium at a concentration of at least about 1×10¹⁹ atoms/cm³, furtherwherein the pulling conditions are sufficient to yield a concentrationof interstitial oxygen in the main body of the single crystal siliconingot of less than about 6 ppma (New ASTM: ASTM F 121, 1980-1983; DIN50438/1, 1978), and further wherein the main body of the single crystalsilicon ingot has a resistivity of at least about 1000 ohm cm.

In one embodiment, the present invention is further directed to a singlecrystal silicon ingot comprising: a central axis, a crown, an endopposite the crown, and a main body between the crown and the oppositeend, the main body having a lateral surface and a radius, R, extendingfrom the central axis to the lateral surface, wherein the main body ofthe single crystal silicon ingot comprises an impurity comprisingnitrogen at a concentration of at least about 1×10¹⁴ atoms/cm³,germanium at a concentration of at least about 1×10¹⁹ atoms/cm³, or acombination nitrogen at a concentration of at least about 1×10¹⁴atoms/cm³ and germanium at a concentration of at least about 1×10¹⁹atoms/cm³, and interstitial oxygen at a concentration of less than about6 ppma (New ASTM: ASTM F 121, 1980-1983; DIN 50438/1, 1978), and furtherwherein the main body of the single crystal silicon ingot has aresistivity of at least about 1000 ohm cm.

Various refinements exist of the features noted in relation to theabove-mentioned aspect. Further features may also be incorporated in theabove-mentioned aspect as well. These refinements and additionalfeatures may exist individually or in any combination. For instance,various features discussed below in relation to any of the illustratedembodiments may be incorporated into the above-described aspect, aloneor in any combination.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of a crucible of one embodiment.

FIG. 2 is a side view of the crucible shown in FIG. 1 .

FIG. 3 is a schematic illustrating a cusped magnetic field applied to acrucible containing a melt in a crystal growing apparatus.

FIG. 4 is a block diagram of a crystal growing system of same embodimentas FIG. 1 .

FIG. 5A is a cross-sectional view of a portion of a crucible showingflowlines and oxygen concentration near the crucible wall atintermediate body growth at a given crystal rotation rate.

FIG. 5B is a cross-sectional view of a portion of an example cruciblemapping flowlines and oxygen concentration near the crucible wall atlate body growth at a crystal rotation rate.

FIG. 5C is a cross-sectional view of a portion of a crucible mappingflowlines and oxygen concentration near the crucible wall at late bodygrowth at a different crystal rotation rate.

FIG. 6 is a graph plotting a simulated oxygen concentration (Oi) as afunction of crystal rotation rate at late body growth versus position(BL) along the crystal.

FIG. 7A is a graph plotting an oxygen concentration at late body growthversus crucible rotation rate for a crystal body rotation rate of 6 rpm.

FIG. 7B is a graph plotting an oxygen concentration at late body growthversus crucible rotation rate for a crystal body rotation rate of 8 rpm.

FIG. 8A is a cross-sectional view of an example crucible mappingflowlines and velocity magnitudes near a crucible wall at late bodygrowth at a magnetic field strength corresponding to 50% balanced.

FIG. 8B is a cross-sectional view of an example crucible mappingflowlines and velocity magnitudes near a crucible wall at late bodygrowth at a magnetic field strength corresponding to 95% balanced.

FIG. 8C is a cross-sectional view of an example crucible mappingflowlines and velocity magnitudes near a crucible wall at late bodygrowth at a magnetic field strength corresponding to 150% balanced.

FIG. 9 is a graph plotting oxygen concentration as a function of crystalbody length for two different crystal rotation rate profiles.

FIGS. 10A and 10B are graphs depicting the no-slip temperature windowtest according to Example 3.

FIG. 11 is a graph depicting the no-slip temperature window testaccording to Example 4.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION OF THE EMBODIENT(S) OF THE INVENTION

The method of the present invention is directed to the growth of singlecrystal silicon ingots under conditions sufficient to product ingotshaving low oxygen concentration, high resistivity, and improvedmechanical strength. The present invention is further directed to singlecrystal silicon ingots produced by the method, and is still furtherdirected to single crystal silicon wafers sliced from single crystalsilicon ingots, the wafers having low oxygen concentration, highresistivity, and improved mechanical strength.

According to some embodiments of the present invention, the crystalgrowth conditions are sufficient to prepare a single crystal siliconingot by the Czochralski method, the ingots comprising germanium (Ge)dopant at a concentration of at least about 1×10¹⁹ atoms/cm³ and havinginterstitial oxygen (Oi) at a concentration of less than about 6.0 ppma.According to some embodiments of the present invention, the crystalgrowth conditions are sufficient to prepare a single crystal siliconingot by the Czochralski method, the ingots comprising nitrogen (N)dopant at a concentration of at least about 1×10¹⁴ atoms/cm³ and havinginterstitial oxygen (Oi) at a concentration of less than about 6.0 ppma.According to some embodiments of the present invention, the crystalgrowth conditions are sufficient to prepare a single crystal siliconingot by the Czochralski method, the ingots comprising nitrogen (N)dopant at a concentration of at least about 1×10¹⁴ atoms/cm³ andgermanium (Ge) dopant at a concentration of at least about 1×10¹⁹atoms/cm³ and having interstitial oxygen (Oi) at a concentration of lessthan about 6.0 ppma. Further, the single crystal silicon ingots, andwafers sliced therefrom, have a resistivity of at least about 1000 ohmcm. In preferred embodiments, the ingots are pulled according to theCzochralski (Cz) batch crystal pulling process. Doping a single crystalsilicon ingot with germanium advantageously improves the mechanicalstrength of the ingot, while also not impacting the resistivity of theSi. In some embodiments, nitrogen can be co-doped with Ge to improvemechanical strength without much impact on resistivity. Theconcentration of Ge and/or N dopant is low enough that the wafer ischaracterized by low or no thermal donor generation during a thermalprocess. Additionally, Ge doping does not degrade the second orderharmonic performance, which can be degraded by other dopants, such asnitrogen, boron, or metals.

In some embodiments, resistivity may be controlled by selecting highpurity polycrystalline silicon with >1000 ohm·cm resistivity and bypreparing the melt in a high purity synthetic quartz lined crucible. Insome embodiments, a compensating P/N type small quantity dilute dopantmay be added to compensate for electrically active impurity to achievedesired resistivity of the crystal. Accordingly, the crystal pullingconditions and materials may be selected to provide a single crystalsilicon ingot wherein the main body of the single crystal silicon ingothas a resistivity of at least about 1000 ohm cm, at least about 3000 ohmcm, at least about 4000 ohm cm, at least about 5000 ohm cm, at leastabout 10000 ohm cm, at least about 15000 ohm cm, or even at least about20000 ohm cm.

In some embodiments, the interstitial oxygen, Oi, may be controlled towithin a required range by puller process optimization. The methodologyfor manufacturing very low Oi single crystal silicon ingots may includecombinations of three key process mechanisms. The three key mechanismsinclude 1) optimization of crucible inner wall temperature and crucibledissolution, 2) transport of Oi from crucible wall to the growingcrystal, and 3) evaporation of SiO from the melt surface to gas phase.The three mechanisms stated above depend strongly on the melt flowconditions established and influenced heavily by the applied magneticfield. In some embodiments, the pulling conditions are sufficient toyield a concentration of interstitial oxygen in the main body of thesingle crystal silicon ingot of less than about 6 ppma, such as lessthan about 5 ppma, less than about 4 ppma, or even less than about 3ppma. These concentrations are according to the New ASTM: ASTM F 121,1980-1983; DIN 50438/1, 1978.

In some embodiments, germanium may be incorporated into the singlecrystal silicon ingot by adding a source of germanium, e.g., elementalgermanium and/or silicon germanium, to the melt during meltdown process.Germanium is thereby incorporated in the single crystal silicon ingotbased on segregation principle. Accordingly, in some embodiments,sufficient germanium, e.g., elemental germanium and/or silicongermanium, may be added to the silicon melt to thereby pull a singlecrystal silicon ingot wherein the main body of the single crystalsilicon ingot comprises germanium at a concentration of at least about1×10¹⁹ atoms/cm³, such as at least about 3×10¹⁹ atoms/cm³, or at leastabout 5×10¹⁹ atoms/cm³. The main body of the single crystal siliconingot may comprise germanium at a concentration of less than about1×10²² atoms/cm³, such as less than about 1×10²¹ atoms/cm³, or less thanabout 1×10²² atoms/cm³. In some embodiments, the main body of the singlecrystal silicon ingot comprises germanium at a concentration of at leastabout 1×10¹⁹ atoms/cm³ and less than about 1×10²² atoms/cm³. In someembodiments, the main body of the single crystal silicon ingot comprisesgermanium at a concentration of at least about 3×10¹⁹ atoms/cm³ and lessthan about 1×10²² atoms/cm³, such as between about 5×10¹⁹ atoms/cm³ andless than about 1×10²¹ atoms/cm³, or between about 5×10¹⁹ atoms/cm³ andless than about 1×10²⁰ atoms/cm³.

In some embodiments, nitrogen may be incorporated into the singlecrystal silicon ingot by adding a source of nitrogen, e.g., siliconnitrogen or nitrogen gas, to the melt during meltdown process. Nitrogenis thereby incorporated in the single crystal silicon ingot based onsegregation principle. In some embodiments, the main body of the singlecrystal silicon ingot comprises nitrogen at a concentration of at leastabout 1×10¹⁴ atoms/cm³, such as at least about 2×10¹⁴ atoms/cm³, or atleast about 5×10¹⁴ atoms/cm³. In some embodiments, the main body of thesingle crystal silicon ingot comprises nitrogen at a concentration of atleast about 1×10¹⁴ atoms/cm³ and less than about 1×10¹⁶ atoms/cm³. Insome embodiments, the main body of the single crystal silicon ingotcomprises nitrogen at a concentration between about 2×10¹⁴ atoms/cm³ andless than about 2×10¹⁵ atoms/cm³, such as at least about 5×10¹⁴atoms/cm³ and less than about 1×10¹⁶ atoms/cm³. In some embodiments, themain body of the single crystal silicon ingot comprises nitrogen at aconcentration of at least about 1×10¹⁵ atoms/cm³ and less than about1×10¹⁶ atoms/cm³.

In some embodiments, germanium may be incorporated into the singlecrystal silicon ingot by adding a source of germanium, e.g., elementalgermanium and/or silicon germanium, and nitrogen may be incorporatedinto the single crystal silicon ingot by adding a source of nitrogen,e.g., silicon nitrogen or nitrogen gas, to the melt during meltdownprocess. Accordingly, in some embodiments, sufficient germanium, e.g.,elemental germanium and/or silicon germanium, may be added to thesilicon melt to thereby pull a single crystal silicon ingot wherein themain body of the single crystal silicon ingot comprises germanium at aconcentration of at least about 1×10¹⁹ atoms/cm³, such as at least about3×10¹⁹ atoms/cm³, or at least about 5×10¹⁹ atoms/cm³, and the main bodyof the single crystal silicon ingot comprises nitrogen at aconcentration of at least about 1×10¹⁴ atoms/cm³, such as at least about2×10¹⁴ atoms/cm³, or at least about 5×10¹⁴ atoms/cm³. In someembodiments, the main body of the single crystal silicon ingot comprisesgermanium at a concentration of at least about 1×10¹⁹ atoms/cm³ and lessthan about 1×10²² atoms/cm³ and nitrogen at a concentration of at leastabout 1×10¹⁴ atoms/cm³ and less than about 1×10¹⁶ atoms/cm³. In someembodiments, the main body of the single crystal silicon ingot comprisesgermanium at a concentration of at least about 3×10¹⁹ atoms/cm³ and lessthan about 1×10²² atoms/cm³ and nitrogen at a concentration of at leastabout 2×10¹⁴ atoms/cm³ and less than about 1×10¹⁵ atoms/cm³.

With reference to FIGS. 1 and 2 , a crucible of one embodiment isindicated generally at 10. A cylindrical coordinate system for crucible10 includes a radial direction R 12, an angular direction θ14, and anaxial direction Z 16. Coordinates R 12, θ14, and Z 16 are used herein todescribe methods and systems for producing low oxygen silicon ingots.The crucible 10 contains a melt 25 having a melt surface 36. A crystal27 is grown from the melt 25. The melt 25 may contain one or moreconvective flow cells 17, 18 induced by heating of the crucible 10 androtation of the crucible 10 and/or crystal 27 in the angular directionθ14. The structure and interaction of these one or more convective flowcells 17, 18 are modulated via regulation of one of more processparameters to reduce the inclusion of oxygen within the forming crystal27 as described in detail herein below. The crucible wall 103 (see FIGS.5A-5C and 8A-8C) may be lined with high purity quartz in order toenhance control of the resistivity. In some embodiments, the crucible 10may be a synthetic crucible comprising a high purity quartz lining thecrucible wall 103.

With Reference to FIG. 3 , a block diagram illustrates a cusped magneticfield being applied to crucible 10 containing melt 23 in a crystalgrowing apparatus. As shown, crucible 10 contains silicon melt 23 fromwhich a crystal 27 is grown. The cusped magnetic field configuration isdesigned to overcome deficiencies of axial and horizontal magnetic fieldconfigurations. A pair of coils 31 and 33 (e.g., Helmholtz coils) areplaced coaxially above and below melt surface 36. Coils 31 and 33 areoperated in an opposed current mode to generate a magnetic field thathas a purely radial field component (i.e., along R 12) near melt surface36 and a purely axial field component (i.e., along Z 16) near an axis ofsymmetry 38 of crystal 27. The combination of an upper magnetic field 40and a lower magnetic field 42 produced by coils 31 and 33, respectively,results in axial and radial cusped magnetic field components.

FIG. 4 is a block diagram of a crystal growing system 100. The crystalgrowing system 100, elements of the crystal growing system 100, andvarious operating parameters of the crystal growing system 100 aredescribed in additional detail in International Application No.PCT/US2014/039164 (Published as WO 2014/190165), which is incorporatedby reference herein in its entirety. Referring again to FIG. 4 , system100 employs a Czochralski crystal growth method to produce asemiconductor ingot. In this embodiment, system 100 is configured toproduce a cylindrical semiconductor ingot having an ingot diameter ofgreater than one-hundred fifty millimeters (150 mm), more specificallyin a range from approximately 150 mm to 460 mm, and even morespecifically, a diameter of approximately three-hundred millimeters (300mm). In other embodiments, system 100 is configured to produce asemiconductor ingot having a two-hundred millimeter (200 mm) ingotdiameter or a four-hundred and fifty millimeter (450 mm) ingot diameter.In addition, in one embodiment, system 100 is configured to produce asemiconductor ingot with a total ingot length of at least five hundredmillimeters (500 mm), such as at least 900 mm. In other embodiments,system 100 is configured to produce a semiconductor ingot with a totalingot length ranging from approximately five hundred millimeters (500mm) to three thousands millimeters (3000 mm), such as between 900 mm and1200 mm.

Referring again to FIG. 4 , the crystal growing system 100 includes avacuum chamber 101 enclosing crucible 10. A side heater 105, forexample, a resistance heater, surrounds crucible 10. A bottom heater106, for example, a resistance heater, is positioned below crucible 10.During heating and crystal pulling, a crucible drive unit 107 (e.g., amotor) rotates crucible 10, for example, in the clockwise direction asindicated by the arrow 108. Crucible drive unit 107 may also raiseand/or lower crucible 10 as desired during the growth process. Withincrucible 10 is silicon melt 25 having a melt level or melt surface 36.In operation, system 100 pulls a single crystal 27, starting with a seedcrystal 115 attached to a pull shaft or cable 117, from melt 25. One endof pull shaft or cable 117 is connected by way of a pulley (not shown)to a drum (not shown), or any other suitable type of lifting mechanism,for example, a shaft, and the other end is connected to a chuck (notshown) that holds seed crystal 115 and crystal 27 grown from seedcrystal 115.

Crucible 10 and single crystal 27 have a common axis of symmetry 38.Crucible drive unit 107 can raise crucible 10 along axis 38 as the melt25 is depleted to maintain melt level 36 at a desired height. A crystaldrive unit 121 similarly rotates pull shaft or cable 117 in a direction110 opposite the direction in which crucible drive unit 107 rotatescrucible 10 (e.g., counter-rotation). In embodiments using iso-rotation,crystal drive unit 121 may rotate pull shaft or cable 117 in the samedirection in which crucible drive unit 107 rotates crucible 10 (e.g., inthe clockwise direction). Iso-rotation may also be referred to as aco-rotation. In addition, crystal drive unit 121 raises and lowerscrystal 27 relative to melt level 36 as desired during the growthprocess.

According to the Czochralski single crystal growth process, a quantityof polycrystalline silicon, or polysilicon, is charged to crucible 10.Additionally, the polycrystalline silicon charge comprises a source ofgermanium, which may be elemental germanium or silicon germanium, asource of nitrogen, which may be nitrogen gas or silicon nitride, orboth a source of germanium and a source of nitrogen in order to dope thesingle crystal silicon ingot pulled from the melt. Suitable sources ofgermanium include elemental germanium and silicon germanium. In someembodiments, elemental, pure germanium is purified by a float zoneprocess. The float zone purified Germanium is crushed into smallchips/chunks and then used to dope the silicon melt. Silicon germaniummay comprise germanium content in a molar ratio generally from about 0.1to about 0.9. Accordingly, in some embodiments, sufficient Germanium,e.g., elemental germanium and/or silicon germanium, may be added to thesilicon melt to thereby pull a single crystal silicon ingot wherein themain body of the single crystal silicon ingot comprises germanium at aconcentration of at least about 1×10¹⁹ atoms/cm³, such as at least about3×10¹⁹ atoms/cm³, or at least about 5×10¹⁹ atoms/cm³. The main body ofthe single crystal silicon ingot may comprise germanium at aconcentration of less than about 1×10²² atoms/cm³, such as less thanabout 1×10²¹ atoms/cm³, or less than about 1×10²⁰ atoms/cm³. Sufficientgermanium is added to the silicon melt so that the resultant main bodyof the single crystal silicon ingot comprises germanium at aconcentration of at least about 1×10¹⁹ atoms/cm³ and less than about1×10²² atoms/cm³, such as at least about 3×10¹⁹ atoms/cm³, or betweenabout 5×10¹⁹ atoms/cm³ and less than about 1×10²¹ atoms/cm³, or betweenabout 5×10¹⁹ atoms/cm³ and less than about 1×10²⁰ atoms/cm³.

In some embodiments, the polycrystalline silicon charge comprises asource of nitrogen, e.g., silicon nitride and/or nitrogen gas. In someembodiments, sufficient nitrogen may be added so that the main body ofthe single crystal silicon ingot comprises nitrogen at a concentrationof at least about 1×10¹⁴ atoms/cm³ and less than about 1×10¹⁶ atoms/cm³.In some embodiments, the main body of the single crystal silicon ingotcomprises nitrogen at a concentration between about 2×10¹⁴ atoms/cm³ andless than about 2×10¹⁵ atoms/cm³. In some embodiments, the main body ofthe single crystal silicon ingot comprises nitrogen at a concentrationof at least about 5×10¹⁴ atoms/cm³ and less than about 1×10¹⁶ atoms/cm³.In some embodiments, the main body of the single crystal silicon ingotcomprises nitrogen at a concentration of at least about 1×10¹⁵ atoms/cm³and less than about 1×10¹⁶ atoms/cm³.

In some embodiments, the polycrystalline silicon charge comprises asource of germanium and a source of nitrogen. Accordingly, in someembodiments, sufficient germanium, e.g., elemental germanium and/orsilicon germanium, may be added to the silicon melt to thereby pull asingle crystal silicon ingot wherein the main body of the single crystalsilicon ingot comprises germanium at a concentration of at least about1×10¹⁹ atoms/cm³, such as at least about 3×10¹⁹ atoms/cm³, or at leastabout 5×10¹⁹ atoms/cm³, and sufficient nitrogen may be added to thesilicon melt to thereby pull a single crystal silicon ingot comprisingnitrogen at a concentration of at least about 1×10¹⁴ atoms/cm³, such asat least about 2×10¹⁴ atoms/cm³, or at least about 5×10¹⁴ atoms/cm³.

A heater power supply 123 energizes resistance heaters 105 and 106, andinsulation 125 lines the inner wall of vacuum chamber 101. A gas supply127 (e.g., a bottle) feeds argon gas to vacuum chamber 101 via a gasflow controller 129 as a vacuum pump 131 removes gas from vacuum chamber101. An outer chamber 133, which is fed with cooling water from areservoir 135, surrounds vacuum chamber 101.

The cooling water is then drained to a cooling water return manifold137. Typically, a temperature sensor such as a photocell 139 (orpyrometer) measures the temperature of melt 25 at its surface, and adiameter transducer 141 measures a diameter of single crystal 27. Inthis embodiment, system 100 does not include an upper heater. Thepresence of an upper heater, or lack of an upper heater, alters coolingcharacteristics of crystal 27.

An upper magnet, such as solenoid coil 31, and a lower magnet, such assolenoid coil 33, are located above and below, respectively, melt level36 in this embodiment. The coils 31 and 33, shown in cross-section,surround vacuum chamber (not shown) and share axes with axis of symmetry38. In one embodiment, the upper and lower coils 31 and 33 have separatepower supplies, including, but not limited to, an upper coil powersupply 149 and a lower coil power supply 151, each of which is connectedto and controlled by control unit 143.

In this embodiment, current flows in opposite directions in the twosolenoid coils 31 and 33 to produce a magnetic field (as shown in FIG. 3). A reservoir 153 provides cooling water to the upper and lower coils31 and 33 before draining via cooling water return manifold 137. Aferrous shield 155 surrounds coils 31 and 33 to reduce stray magneticfields and to enhance the strength of the field produced.

A control unit 143 is used to regulate the plurality of processparameters including, but not limited to, at least one of crystalrotation rate, crucible rotation rate, and magnetic field strength. Invarious embodiments, the control unit 143 may include a processor 144that processes the signals received from various sensors of the system100 including, but not limited to, photocell 139 and diameter transducer141, as well as to control one or more devices of system 100 including,but not limited to: crucible drive unit 107, crystal drive unit 121,heater power supply 123, vacuum pump 131, gas flow controller 129 (e.g.,an argon flow controller), upper coil power supply 149, lower coil powersupply 151, and any combination thereof.

In the example embodiment, system 100 produces single crystal siliconingots suitable for use in device manufacturing. The single crystalsilicon ingot is generally cylindrical and, due to pulling conditions,is capped with a conical crown and a conical end opposite the crown.Accordingly, a single crystal silicon ingot pulled according to themethod of the present invention comprises a central axis, a crown, anend opposite the crown, and a main body between the crown and theopposite end, the main body having a lateral surface and a radius, R,extending from the central axis to the lateral surface. Advantageously,system 100 may be used to produce single crystal silicon ingot 27, asubstantial portion or all of which is substantially free ofagglomerated intrinsic point defects. Furthermore, system 100 may beused to produce single crystal silicon ingot 27 having substantially noagglomerated defects that are larger than approximately one hundredtwenty nanometers (nm) in diameter, or more particularly, approximatelyninety nm in diameter. The shape of the melt-solid or melt-crystalinterface and the pull speed is controlled during crystal growth tolimit and/or suppress the formation of agglomerated intrinsic pointdefects.

During production, oxygen is introduced into single crystal siliconingots through the melt-solid or melt crystal interface. However, oxygenmay cause various defects in wafers produced from the ingots, reducingthe yield of semiconductor devices. Accordingly, it is desirable toproduce silicon crystal ingots with a low oxygen concentration. Usingthe methods described herein, single crystal silicon ingots are producedhaving an oxygen concentration less than approximately 6 ppma, or lessthan approximately 5 ppma, or less than approximately 4 ppma less thanapproximately 3 ppma. These concentrations are according to the NewASTM: ASTM F 121, 1980-1983; DIN 50438/1, 1978.

Without being limited to any particular theory, oxygen is introducedinto the growing silicon crystal ingot emerging from the melt by aninteracting series of events, each of which is influenced by at leastone process parameter as described herein below. SiO is introduced intothe melt via dissolution at the crucible wall. The SiO introduced at thecrucible wall may be moved elsewhere in the melt via flow induced bybuoyancy forces created by localized heating of the melt neat thecrucible wall. The SiO may be further moved by additional flow inducedby the rotation rate of the crystal at the melt-crystal interface aswell as rotation rate of the crucible itself. The concentration of SiOin the melt may be reduced via evaporation from the melt at the exposedsurface of the melt. The interaction of any combination of dissolution,convection, and evaporation of SiO within the melt influences theconcentration of SiO in the melt situated near the crystal-meltinterface that is formed into the silicon crystal ingot. In variousaspects, any one or more process parameters are simultaneously regulatedto reduce the concentration of SiO situated near the melt-crystalinterface, and consequently reduce the oxygen concentration within thesilicon crystal ingot formed according to the method.

In various embodiments, various process parameters are regulatedsimultaneously to facilitate producing silicon crystal ingots with a lowoxygen concentration. In one embodiment, the various process parametersare regulated in at least two stages that include an intermediate bodygrowth stage corresponding to growth of the silicon crystal ingot up toan intermediate ingot lengths of approximately 800 mm, and a late bodygrowth stage corresponding to growth of the silicon crystal ingot froman intermediate ingot length of approximately 800 mm up to the totalingot length. In this embodiment, the regulation of the various processparameters in at least two different stages accounts for changes in thenature of the interaction of dissolution, convection, evaporation of SiOwithin the melt, depth of the melt in the crucible, and the flow cellswithin the melt in the crucible as the silicon crystal ingot grows inlength.

In particular, the role of convection is modified over the formation ofthe entire silicon crystal ingot due to a decrease in the depth of themelt within the crucible associated with growth of the silicon crystalingot, as described in detail below. As a result, at the late bodygrowth stage, the regulation of at least one process parameter ismodified differently relative to the regulation of these same parametersat the intermediate body growth stage. In some embodiments, at the latebody growth stage, the regulation of at least three process parametersis modified differently relative to the regulation of these sameparameters at the intermediate body growth stage. As described hereinbelow, the regulation of the process parameters modulate various factorsrelated to the convection of SiO within the melt at the late body growthstage. In one embodiment, the process parameters with modifiedregulation during the late body growth stage include, but are notlimited to: seed rotation rate, crucible rotation rate, and magneticfield strength.

Referring again to FIG. 4 , seed rotation rate refers to the rate atwhich pull shaft or cable 117 rotates seed crystal 115 about axis 38.Seed rotation rate impacts the flow of SiO from crucible 10 to crystal27 and a rate of SiO evaporation from melt 25. Referring again to FIG. 2, the flow of SiO from crucible 10 to crystal 27 is influenced generallyby interactions between crystal flow cell 18 driven by the rotation ofcrystal 27 at the seed rotation rate within melt 25 and buoyancy flowcell 17 driven by heating of melt 25 within crucible 10. The impact ofseed rotation rate on the flow of SiO from crucible 10 to crystal 27differs depending on the stage of growth of crystal 27.

FIG. 5A is a cross-sectional view of simulated flowlines 109 and oxygenconcentrations within melt 25 (with reference to FIG. 2 ) at anintermediate body growth stage, corresponding to growth of crystal 27(with reference to FIG. 1 ) up to an intermediate ingot length ofapproximately 800 mm. At the intermediate body growth stage, depth 200of melt 25 within crucible 10 is sufficiently deep to effectivelydecouple interactions between fluid motion induced by crystal flow cell18 and buoyancy flow cell 17. A high seed rotation rate (i.e. 12 rpm)reduces the boundary layer thickness between melt line 36 and the gasabove melt 25 to increase SiO evaporation. Further, a high seed rotationrate decreases melt flow from crucible 10 to crystal 27 by suppressingbuoyancy flow cell 17 with induced crystal flow cell 18, as illustratedin FIG. 5A. Moreover, a high seed rotation rate creates an outwardradial flow that retards the inward flow (i.e., transport) of SiO fromcrucible 10, reducing the oxygen concentration in crystal 27.

FIG. 5B is a cross-sectional view of simulated flowlines 109 and oxygenconcentrations within melt 25 at a late body growth stage, correspondingto growth of crystal 27 from an intermediate ingot length ofapproximately 800 mm up to the total ingot length. Due to removal ofmelt 25 from crucible 10 associated with formation of crystal 27, depth200 at the late body growth stage is shallower with respect to depth 200at intermediate body growth stage as illustrated in FIG. 5A. At asimilarly high seed rotation rate to that used to perform the simulationillustrated in FIG. 5A (i.e. 12 rpm), crystal flow cell 18 contacts theinner wall of crucible 10, causing convection of SiO formed at the innerwall of crucible 10 into crystal 27 formed at the late body growthstage.

FIG. 5C is a cross-sectional view of simulated flowlines 109 and oxygenconcentrations within melt 25 at a late body growth stage calculated ata lower (e.g., 8 rpm) seed rotation rate. Crystal flow cell 18 inducedby the lower seed rotation rate does not extend to the inner wall ofcrucible 10, but instead is excluded by buoyancy cell 17. As a result,the flow of SiO produced at the inner wall of crucible 10 to crystal 27is disrupted, thereby reducing the oxygen concentration within crystal27 formed at the late body growth stage at reduced seed rotation rate.

As described herein, the transition from an intermediate to a late bodygrowth stage is a soft transition. The transition may vary depending onvarious parameters of the process, such as crucible size, shape, depthof melt, modeling parameters, and the like. Generally, at theintermediate body growth stage, parameters are such that there arelimited or no interactions between fluid motion induced by crystal flowcell 18 and buoyancy flow cell 17; the crystal flow cell 18 and buoyancyflow cell 17 are effectively decoupled. At the late body growth stage,parameters are such that there are interactions between fluid motioninduced by crystal flow cell 18 and buoyancy flow cell 17; the crystalflow cell 18 and buoyancy flow cell 17 are effectively coupled. By wayof non-limiting example, late body growth stage occurs when less thanabout 37% of the initial mass of melt 25 is left in crucible 10 in anembodiment that includes an initial melt mass between 180 kg to 450 kgin a crucible 10 with an inner diameter of about 36 inches. In variousembodiments, depth 200 of melt 25 within crucible 10 is monitored toidentify the transition from the intermediate to a late body growthstage. In other examples, the late body growth stage occurs when lessthan about 35%, less than about 40%, less than about 45%, or less thanabout 50% of the initial mass of melt 25 is left in crucible 10. In someembodiments, the transition from intermediate to late body growth stageis determined based on the depth of melt 25, or any other suitableparameter.

In various embodiments, the method includes regulating the seed rotationrate in at least two stages including, but not limited to, theintermediate body growth stage and the late body growth stage. In oneembodiment, the method includes rotating crystal 27 during theintermediate body growth stage at a seed rotation rate ranging fromapproximately 8 to 14 rpm, and more specifically 12 rpm. In thisembodiment, the method further includes reducing the seed rotation rateat the late body growth stage to a seed rotation rate ranging fromapproximately 6 rpm to 8 rpm, and more specifically 8 rpm.

In another embodiment, the seed rotation rate may be reduced accordingto the intermediate ingot length. By way of non-limiting example, theseed rotation rate may be regulated to approximately 12 rpm forintermediate ingot lengths up to approximately 850 mm, and may befurther regulated to linearly decrease to approximately 8 rpm at anintermediate ingot length of approximately 950 mm, and then regulateseed rotation rate at approximately 8 rpm up to the total ingot length,as illustrated in FIG. 9 . As also illustrated in FIG. 9 , the oxygencontent of the crystal within the body length ranging from approximately800 mm to the total ingot length is reduced compared to a crystal formedat a constant seed rotation rate of approximately 12 rpm. FIG. 6 is agraph comparing the simulated oxygen concentration of crystals formed atseed rotation rates according to three rotation schedules: a) rotationat 12 rpm for the formation of the entire crystal; b) rotation at 12 rpmup to an intermediate crystal length of 900 mm followed by rotation at 8rpm for formation of the remaining crystal length; and c) rotation at 12rpm up to an intermediate crystal length of 900 mm followed by rotationat 6 rpm for formation of the remaining crystal length. As illustratedin FIG. 6 , lower seed rotation rates reduced oxygen concentrationwithin the portion of the crystal formed at the late body growth stage.

Crucible rotation rate may further influence the oxygen concentrationswithin crystals 27 formed according to embodiments of the method.Crucible rotation rate refers to the rate at which crucible 10 isrotated about axis 38 using crucible drive unit 107. Crucible rotationrate impacts the flow of SiO from crucible 10 to crystal 27 and anamount of SiO evaporating from melt 25. A high crucible rotation ratereduces both a boundary layer thickness between crucible 10 and melt 25,and a boundary layer thickness between melt line 36 and the gas abovemelt 25. However, to minimize the oxygen concentration in crystal 27, athicker boundary layer between crucible 10 and melt 25 is desired toreduce the SiO transport rate, while a thinner boundary layer betweenmelt line 36 and the gas above melt 25 is desired to increase the SiOevaporation rate. Accordingly, the crucible rotation rate is selected tobalance the competing interests of a high boundary layer thicknessbetween crucible 10 and melt 25 resulting from slower crucible rotationrates and a low boundary layer thickness between melt line 36 and thegas above melt 25 resulting from higher crucible rotation rates.

Changes in depth 200 of melt 10 between intermediate body growth stageand late body growth stage described herein above influence the impactof modulation of crucible rotation rate on oxygen concentration in amanner similar to the influence of seed rotation rate described hereinpreviously. In various embodiments, the method includes regulating thecrucible rotation rate in at least two stages including, but not limitedto, the intermediate body growth stage and the late body growth stage.In one embodiment, the method includes rotating crucible 10 at theintermediate body growth stage at a crucible rotation rate ranging fromapproximately 1.3 rpm to approximately 2.2, and more specifically 1.7rpm. In this embodiment, the method further includes reducing thecrucible rotation rate at the late body growth stage to a cruciblerotation rate ranging from approximately 0.5 rpm to approximately 1.0rpm, and more specifically 1 rpm.

FIGS. 7A and 7B are graphs showing a simulated oxygen concentrationwithin silicon ingots as a function of the crucible rotation rate atlate body growth stage. The silicon ingots of FIG. 7A were formed usingan embodiment of the method in which the seed rotation rate was reducedfrom 12 rpm to 6 rpm at late body growth stage, and the cruciblerotation rate was reduced from about 1.7 rpm to 1 rpm or 1.5 rpm at latebody growth stage. The silicon ingots of FIG. 7B were formed using anembodiment of the method in which the seed rotation rate was reducedfrom 12 rpm to 8 rpm at late body growth stage, and the cruciblerotation rate was reduced from about 1.7 rpm to 0.5 rpm, 1 rpm, or 1.5rpm at late body growth stage. In both simulations, lower cruciblerotation rates were associated with lower oxygen concentrations withinthe resulting silicon ingot.

The method may further include regulating magnet strength in at leasttwo stages including, but not limited to, the intermediate body growthstage and the late body growth stage. Magnet strength refers to thestrength of the cusped magnetic field within the vacuum chamber. Morespecifically, magnet strength is characterized by a current throughcoils 31 and 33 that is controlled to regulate magnetic strength.Magnetic strength impacts the flow of SiO from crucible 10 to crystal27. That is, a high magnetic strength minimizes the flow of SiO fromcrucible 10 to crystal 27 by suppressing a buoyancy force within melt25. As the magnetic field suppresses the buoyancy flow, it decreases thedissolution rate of the quartz crucible, thus lowering the interstitialoxygen incorporated into the crystal. However, if the magnetic fieldstrength increases beyond a certain level, further retardation in thebuoyancy flow may result in decreasing the evaporation rate at the meltfree surface, thus raising the interstitial oxygen levels. Due todifferences in the relative contribution of buoyancy flow to the oxygencontent of the crystal at the late body formation stage relative to theintermediate body formation stage as described previously herein, anadjustment to the magnet strength at the late body formation stageenables appropriate modulation of buoyancy flow to reduce oxygen withinthe crystal formed at the late body formation stage.

In various embodiments, the method includes regulating the magneticfield strength in at least two stages including, but not limited to, theintermediate body growth stage and the late body growth stage. In oneembodiment, the method includes regulating the magnetic field strengthat the intermediate body growth stage such that the magnetic fieldstrength is approximately 0.02 to 0.05 Tesla (T) at an edge of crystal27 at the melt-solid interface and approximately 0.05 to 0.12 T at thewall of crucible 10. In another aspect, the method includes regulatingthe magnetic field strength at the late body growth stage such that themagnetic field strength is approximately 150% of the magnetic fieldstrength used during the intermediate body growth stage, correspondingto approximately 0.03 to 0.075 Tesla (T) at an edge of crystal 27 at themelt-solid interface and approximately 0.075 to 0.18 T at the wall ofcrucible 10.

FIGS. 8A, 8B, and 8C are cross-sectional views of simulated flowlines109 and total speeds within melt 25 at a late body growth stage. FIG. 8Awas simulated using magnetic field strengths corresponding to 50% of themagnetic field used at intermediate body growth stage (i.e.approximately 0.01 to 0.025 Tesla (T) at an edge of crystal 27 at themelt-solid interface and approximately 0.025 to 0.06 T at the wall ofcrucible 10). FIG. 8B was simulated using magnetic field strengthscorresponding to 95% of the magnetic field used at intermediate bodygrowth stage approximately 0.019 to 0.0475 Tesla (T) at an edge ofcrystal 27 at the melt-solid interface and approximately 0.0475 to 0.114T at the wall of crucible 10. FIG. 8C was simulated using magnetic fieldstrengths corresponding to 150% of the magnetic field used atintermediate body growth stage (i.e. approximately 0.03 to 0.075 Tesla(T) at an edge of crystal 27 at the melt-solid interface andapproximately 0.075 to 0.18 T at the wall of crucible 10). ComparingFIGS. 8A, 8B, and 8C, as the strength of the magnetic field increases,flow 300 from the bottom of crucible 10 to melt-crystal interface 302transitions from relatively high convection to melt-crystal interface302 at low magnetic field strength (FIG. 8A) to a relatively littleconvection at higher magnetic field strengths. This suppression ofbuoyancy flow within melt 25 by the increased magnetic field results inlower oxygen concentration in the resulting silicon ingot, as summarizedin Table 1 below. At 150% magnetic field strength, the simulated oxygenconcentration was within the desired range below 5 parts per millionatoms (ppma).

TABLE 1 Effect of Magnetic Field Strength at Late Body Growth Stage onOxygen Concentration in Silicon Ingots Simulated Magnetic Field StrengthOxygen (% intermediate body growth Concentration stage field strength)(ppma)  50% 9.3  95% 6.4 150% 4.5

One or more additional process parameters may be regulated to facilitateproducing silicon crystal ingots with a low oxygen concentration.However, the effects of these additional process parameters are notsensitive to the changes in the depth 200 of melt 25 within crucible 10during growth of crystal 27. Consequently, the regulation of theadditional process parameters described herein remains essentially thesame between different stages of crystal growth, as described inadditional detail below.

One additional process parameter that is controlled, at least in someembodiments, is wall temperature of crucible 10. The wall temperature ofcrucible 10 corresponds to a dissolution rate of crucible 10.Specifically, the higher the wall temperature of crucible 10, the fasterthat portions of crucible 10 will react with and dissolve into melt 25,generating SiO into the melt and potentially increasing an oxygenconcentration of crystal 27 via the melt-crystal interface. Accordingly,reducing the wall temperature of crucible 10, as used herein, equates toreducing the dissolution rate of crucible 10. By reducing the walltemperature of crucible 10 (i.e., reducing the dissolution rate ofcrucible 10), the oxygen concentration of crystal 27 can be reduced.Wall temperature may be regulated by controlling one or more additionalprocess parameters including, but not limited to heater power and meltto reflector gap.

Heater power is another process parameter that may be controlled in someembodiments to regulate the wall temperature of crucible 10. Heaterpower refers to the power of side and bottom heaters 105 and 106.Specifically, relative to typical heating configurations, by increasinga power of side heater 105 and reducing a power of bottom heater 106, ahot spot on the wall of crucible 10 is raised close to the melt line 36.As the wall temperature of crucible 10 at or below melt line 36 islower, the amount of SiO generated by melt 25 reacting with crucible 10is also lower. The heater power configuration also impacts melt flow byreducing the flow (i.e., transport) of SiO from crucible 10 to singlecrystal 27. In this embodiment, a power of bottom heater 106 isapproximately 0 to 5 kilowatts, and more specifically approximately 0kilowatts, and a power of side heater 105 is in a range fromapproximately 100 to 125 kilowatts. Variations in the power of sideheater 105 may be due to, for example, variation in a hot zone age frompuller to puller.

In some embodiments, melt to reflector gap is an additional processparameter that is controlled to regulate the wall temperature ofcrucible 10. Melt to reflector gap refers to a gap between melt line 36and a heat reflector (not shown). Melt to reflector gap impacts the walltemperature of crucible 10. Specifically, a larger melt to reflector gapreduces the wall temperature of crucible 10. In this embodiment, themelt to reflector gap is between approximately 60 mm and 80 mm, and morespecifically 70 mm.

Seed lift is an additional process parameter that is controlled toregulate the flow of SiO from crucible 10 to crystal 27. Seed liftrefers to the rate at which pull shaft or cable 117 lifts seed crystal115 out of melt 25. In one embodiment, seed crystal 115 is lifted at arate between about 0.4 mm/min and about 0.7 mm/min, for example, in arange of approximately 0.42 to 0.55 millimeters per minute (mm/min), andmore specifically 0.46 mm/min for 300 mm product. This pull rate isslower than pull rates typically used for smaller diameter (e.g., 200mm) crystals. For example, the seed lift for 200 mm product may betweenabout 0.55 mm/min and about 0.95 mm/min, such as in a range ofapproximately 0.55 to 0.85 mm/min, and more specifically 0.7 mm/min.

Pull speed is an additional process parameter that may be regulated tocontrol the defect quality of the crystal. For example, using SP2 laserlight scattering, the detected agglomerated point defects generated bythe process described herein may be less than 400 counts for defectsless than 60 nm, less than 100 counts for defects between 60 and 90 nm,and less than 100 counts for less defects between 90 and 120 nm.

In some embodiments, inert gas flow is an additional process parameterthat is controlled to regulate the SiO evaporation from melt 25. Inertgas flow, as described herein, refers to the rate at which argon gasflows through vacuum chamber 101. Increasing the argon gas flow ratesweeps more SiO gas above melt line 36 away from crystal 27, minimizinga SiO gas partial pressure, and in turn increasing SiO evaporation. Inthis embodiment, the argon gas flow rate is in a range fromapproximately 100 slpm to 150 slpm.

Inert gas pressure is an additional process parameter also controlled toregulate the SiO evaporation from melt 27 in some embodiments. Inert gaspressure, as described herein, refers to the pressure of the argon gasflowing through vacuum chamber 101. Decreasing the argon gas pressureincreases SiO evaporation and hence decreases SiO concentration in melt25. In this embodiment, the argon gas pressure ranges from approximately10 torr to 30 torr.

In suitable embodiments, cusp position is an additional processparameter that is controlled to regulate the wall temperature ofcrucible 10 and the flow of SiO from crucible 10 to crystal 27. Cuspposition, as described herein, refers to the position of the cusp of themagnetic field generated by coils 31 and 33. Maintaining the cuspposition below melt line 36 facilitates reducing the oxygenconcentration. In this embodiment, the cusp position is set in a rangefrom approximately 10 mm to 40 mm below melt line 36, more specifically,in a range of approximately 25 mm to 35 mm below melt line 36, and evenmore specifically, at approximately 30 mm.

By controlling process parameters (i.e., heater power, crucible rotationrate, magnet strength, seed lift, melt to reflector gap, inert gas flow,inert gas pressure, seed rotation rate, and cusp position) as describedabove, a plurality of process parameters (i.e., a wall temperature of acrucible, a flow of SiO from the crucible to a single crystal, and anevaporation of SiO from a melt) are regulated to produce single crystalsilicon ingots having a low oxygen concentration. In one embodiment, themethods described herein facilitate producing a silicon ingot with aningot diameter greater than approximately 150 millimeters (mm), a totalingot length of at least approximately 900 mm, and an oxygenconcentration less than 6 ppma, such as less than about 5 ppma, lessthan about 4 ppma, or even less than about 3 ppma. In anotherembodiment, the methods described herein facilitate producing a siliconingot with an ingot diameter ranging from approximately 150 mm to 460mm, specifically approximately 300 mm, and an oxygen concentration lessthan 6 ppma, such as less than about 5 ppma, less than about 4 ppma, oreven less than about 3 ppma. In another additional embodiment, themethods described herein facilitate producing a silicon ingot with atotal ingot length ranging from approximately 900 mm to 1200 mm, and anoxygen concentration less than 6 ppma, such as less than about 5 ppma,less than about 4 ppma, or even less than about 3 ppma. Theseconcentrations are according to the New ASTM: ASTM F 121, 1980-1983; DIN50438/1, 1978.

A single crystal silicon wafer may be sliced according to conventionaltechniques from a single crystal silicon ingot or slug preparedaccording to the method of the present invention. In general, a singlecrystal silicon wafer comprises two major, parallel surfaces, one ofwhich is a front surface of the single crystal silicon wafer and theother of which is a back surface of the single crystal silicon wafer, acircumferential edge joining the front and back surfaces of the singlecrystal silicon wafer, a bulk region between the front and backsurfaces, and a central plane of the single crystal silicon waferbetween the front and back surfaces of the single crystal silicon wafer.Wafers then undergo conventional processing. Accordingly, any sharp,fragile edges are rounded or “profiled” to provide strength andstability to the wafer. This will ultimately prevent chipping orbreakage in subsequent processing. Next, each wafer is laser-marked withvery small alphanumeric or bar code characters. This laser-mark ID givesfull trace-ability to the specific date, machine, and facility where thewafers were manufactured. The wafers are then loaded into a precision“lapping” machine that uses pressure from rotating plates and anabrasive slurry to ensure a more uniform, simultaneous removal of sawdamage present on both front and backside surfaces. This step alsoprovides stock removal and promotes flatness uniformity. Now the wafersmust go through an “etching” cycle. Chemical etching is necessary forthe removal of residual surface damage caused by lapping; it alsoprovides some stock removal. During the etching cycle, wafers progressdown another series of chemical baths and rinse tanks with precise fluiddynamics. These chemical solutions produce a flatter, stronger waferwith a glossy finish. All wafers are then sampled for mechanicalparameters and for process feedback.

Single crystal silicon wafers having low oxygen concentration (i.e.,less than about 6 ppma, less than about 5 ppma, less than about 4 ppma,or even less than about 3 ppma) using the systems and methods describedherein may be advantageous in a variety of applications. For example,insulated-gate bipolar transistors (IGBTs), high quality radio-frequency(RF), high resistivity silicon on insulator (HR-SOI), charge trap layerSOI (CTL-SOI), and substrate for GaN EPI applications may benefit fromthe low oxygen concentration because they achieve high resistivity anddo not have p-n junctions. In some embodiments, the resistivity of asingle crystal silicon wafer is at least about 3000 ohm cm, such as atleast about 4000 ohm cm, at least about 5000 ohm cm, at least about10000 ohm cm, such as at least about 15000 ohm cm, or even at leastabout 20000 ohm cm. Wafers produced for IGBT applications using themethods described herein may, for example, have 30 to 300 ohm-centimeter(ohm-cm) N-type resistivity or greater than 750 ohm-cm N/P-typeresistivity. Further, wafers produced for radiofrequency (RF), highresistivity silicon on insulator (HR-SOI), charge trapping layer SOI(CTL-SOI). and/or GaN EPI applications using the methods describedherein may have, for example, greater than 750 ohm-cm P-type wafers, orat least about 3000 ohm cm, such as at least about 4000 ohm cm, at leastabout 5000 ohm cm, at least about 10000 ohm cm, such as at least about15000 ohm cm, or even at least about 20000 ohm cm. Wafers produced bythe described systems and methods may also be used as handle wafers. ForP-type wafers produced using the methods described herein, boron,aluminum, gallium, and/or indium may be suitably used has a majoritycarrier, and red phosphorus, phosphorus, arsenic, and/or antimony may beused as a minority carrier. For N-type wafers produced using the methodsdescribed herein, red phosphorus, phosphorus, arsenic, and/or antimonymay be used as the majority carrier, and boron, aluminum, gallium,and/or indium may be used as the minority carrier.

To improve mechanical strength and slip performance, wafers producedusing the methods described are co-doped (e.g., by doping the singlecrystal that forms the ingot) with germanium and/or nitrogen. Germaniummay be incorporated into the single crystal silicon ingot by adding asource of germanium, e.g., elemental germanium and/or silicon germanium,to the melt during meltdown process. Germanium is thereby incorporatedin the solid crystal form based on segregation principle. Accordingly,in some embodiments, single crystal silicon wafers sliced from ingotspulled according to the method of the present invention comprisegermanium at a concentration of at least about 1×10¹⁹ atoms/cm³, such asat least about 3×10¹⁹ atoms/cm³, or at least about 5×10¹⁹ atoms/cm³. Thesingle crystal silicon wafers sliced from ingots pulled according to themethod of the present invention may comprise germanium at aconcentration of less than about 1×10²² atoms/cm³, such as less thanabout 1×10²¹ atoms/cm³, or less than about 1×10²⁰ atoms/cm³. In someembodiments, the single crystal silicon wafers comprise germanium at aconcentration of at least about 1×10¹⁹ atoms/cm³ and less than about1×10²² atoms/cm³. In some embodiments, the single crystal silicon waferscomprise germanium at a concentration of at least about 5×10¹⁹ atoms/cm³and less than about 1×10²² atoms/cm³. Single crystal silicon waferssliced from the main body portion of the single crystal silicon ingotscomprises germanium within these concentration ranges.

In some embodiments, nitrogen may be incorporated into the singlecrystal silicon ingot by adding a source of nitrogen, e.g., siliconnitrogen or nitrogen gas, to the melt during meltdown process. Inaddition to improving mechanical strength, nitrogen dopant may interactwith Oi in order to partially alter the electrical property of thewafers by forming thermal donors. This electrical property may be afunction of temperature, as the generation and killing temperature ofsuch species are different for each individual species. The thermaldonor generated by O—O, N—O, Ge—O have different thermal stability. Forexample, N—O thermal donor is found to be generated around 600° C. andmay be stable up <900° C. Thus to dissociate the N—O thermal donor,a >900° C. thermal donor kill (TDK) step may be required to control theresistivity of the wafers. Alternatively, in absence of such TDK processstep, the doping of the crystal may be adjusted to compensate for theresistivity shift by TD, to achieve target resistivity. In someembodiments, the single crystal silicon ingot is co-doped with germaniumand nitrogen. In some embodiments, the main body of the single crystalsilicon ingot comprises germanium at a concentration of at least about1×10¹⁹ atoms/cm³, at least about 3×10¹⁹ atoms/cm³, such as at leastabout 5×10¹⁹ atoms/cm³ and nitrogen at a concentration of at least about1×10¹⁴ atoms/cm³. In some embodiments, the main body of the singlecrystal silicon ingot comprises germanium at a concentration of at leastabout 1×10¹⁹ atoms/cm³ and less than about 1×10²² atoms/cm³ and nitrogenat a concentration of at least about 1×10¹⁴ atoms/cm³ and less thanabout 1×10¹⁶ atoms/cm³. In some embodiments, the main body of the singlecrystal silicon ingot comprises nitrogen at a concentration of at leastabout 5×10¹⁴ atoms/cm³ and less than about 1×10¹⁶ atoms/cm³. In someembodiments, the main body of the single crystal silicon ingot comprisesnitrogen at a concentration of at least about 2×10¹⁴ atoms/cm³ and lessthan about 1×10¹⁵ atoms/cm³. In some embodiments, the main body of thesingle crystal silicon ingot comprises germanium at a concentration ofat least about 3×10¹⁹ atoms/cm³ and less than about 1×10²² atoms/cm³ andnitrogen at a concentration of at least about 1×10¹⁵ atoms/cm³ and lessthan about 1×10¹⁶ atoms/cm³. Single crystal silicon wafers sliced fromthe main body portion of the single crystal silicon ingots comprisegermanium and nitrogen within these concentration ranges.

Example systems and methods of producing single crystal silicon ingotswith relatively low oxygen concentration from a melt formed frompolycrystalline silicon are described herein. These methods takeadvantage of changes in the structure of flow cells in the melt betweena first and second stage of production of the ingot to producerelatively low oxygen silicon. During the first stage, the silicon ingotis relatively small and the depth of the melt is relatively deep. Thesecond stage is characterized by a depleted melt depth within thecrucible due to formation of the silicon ingot. In this second stage, aflow cell induced by rotation of the silicon ingot within the melt maycontact the bottom of the crucible, causing unwanted inclusion ofsilicon oxide formed at the crucible bottom into the growing crystalingot. The methods and systems described herein control production ofthe ingot to limit the including of the unwanted silicon oxide.Generally, at least one process parameter is changed during the secondstage relative to its value during the first stage. Non-limitingexamples of changes in process parameters from the first stage to thesecond stage include: reduced crystal rotation rate, reduced cruciblerotation rate, increased magnetic field strength, and any combinationthereof. For example, in some embodiments, the silicon ingot is rotatedmore slowly during the second stage to reduce contact of the rotationinduced flow cell with the bottom of the crucible, and thereby reducethe amount oxygen included in the silicon ingot.

The systems and methods described herein enable the formation of singlecrystal silicon ingots with low oxygen concentration maintained over alonger ingot length than was achieved using previous methods. A detaileddescription of the effects of these changes in process parameters on thestructure of flow cells within the crucible and the oxygen content ofthe silicon ingots formed using the method on various embodiments, aredescribed in further detail herein.

Embodiments of the methods described herein achieve superior resultscompared to prior methods and systems. For example, the methodsdescribed herein facilitate producing silicon ingots with a lower oxygenconcentration than at least some known methods. Further, unlike at leastsome known methods, the methods described herein may be used for theproduction of ingots having a diameter greater than 150 mm, such asabout 300 mm.

Additionally, incorporation of germanium impurity and/or nitrogenimpurity, into the low oxygen content, high resistivity wafers improvesmechanical strength in wafers normally suffering from degradedmechanical strength and susceptible to wafer slip during hightemperature operations. The fracture strength of germanium doped lowoxygen wafers may be improved both as grown and post anneal. Anotheradvantage of germanium doping is lowered thermal donor formation,thereby reducing the free carrier concentration and enabling higherresistivity wafers. Still further, germanium doping in single crystalsilicon ingots and wafers can effectively suppress void defects. TheGermanium dopant atoms combine with vacancies, thereby suppressingvacancy aggregation necessary to form voids. An additional effect ofvacancy combination is strain relaxation originating from the mismatchof Ge atoms in silicon crystal lattice. The reduction of free vacanciesleads to suppression of grown-in voids and decreases the formationtemperature and thus the size of voids, leading to poorer thermalstability of voids. In addition, germanium doping in single crystalsilicon ingots and wafers improves the formation of denuded zone (DZ,i.e. a region of low or no oxygen precipitation) in the near surfaceregion can be achieved under one-step high temperature annealing, as aresult of germanium enhancing oxygen precipitation in the bulk regionand the out-diffusion of oxygen in the near surface region.

In addition, both N and Ge doping in Si crystals can effectivelysuppress void defects and reduce the concentration of free vacanciessignificantly (J. Cry. Growth 243 (2002) 371-374). Thus during crystalgrowth, it can be pulled faster for the same defect concentrationcompared to conventional processes, thus improving throughput of theprocess. This is particularly helpful for a charge trap layer SOIapplication (CTL-SOI), where, a poly-silicon based charge trap layer isadded between high resistivity low oxygen handle and P type donor wafer.Due to the presence of poly-silicon layer, high density of crystaldefect (COP) may be allowed on the handle wafers without disruption ofthe process flow during device manufacturing (in particular during LLSinspection).

Wafers sliced from ingots prepared according to the method of thepresent invention are suitable for use as a handle wafer and/or a donorwafer in the manufacture of silicon on insulator structures.Semiconductor wafers (e.g., silicon wafers) may be utilized in thepreparation of composite layer structures. A composite layer structure(e.g., a semiconductor-on-insulator, and more specifically, asilicon-on-insulator (SOI) structure) generally comprises a handle waferor layer, a device layer, and an insulating (i.e., dielectric) film(typically an oxide layer) between the handle layer and the devicelayer. Generally, the device layer is between 0.01 and 20 micrometersthick, such as between 0.05 and 20 micrometers thick. Thick film devicelayers may have a device layer thickness between about 1.5 micrometersand about 20 micrometers. Thin film device layers may have a thicknessbetween about 0.01 micrometer and about 0.20 micrometer. In general,composite layer structures, such as silicon-on-insulator (SOI),silicon-on-sapphire (SOS), and silicon-on-quartz, are produced byplacing two wafers in intimate contact, thereby initiating bonding byvan der Waal's forces, followed by a thermal treatment to strengthen thebond. The anneal may convert the terminal silanol groups to siloxanebonds between the two interfaces, thereby strengthening the bond.

After thermal anneal, the bonded structure undergoes further processingto remove a substantial portion of the donor wafer to achieve layertransfer. For example, wafer thinning techniques, e.g., etching orgrinding, may be used, often referred to as back etch SOI (i.e., BESOI),wherein a silicon wafer is bound to the handle wafer and then slowlyetched away until only a thin layer of silicon on the handle waferremains. See, e.g., U.S. Pat. No. 5,189,500, the disclosure of which isincorporated herein by reference as if set forth in its entirety. Thismethod is time-consuming and costly, wastes one of the substrates andgenerally does not have suitable thickness uniformity for layers thinnerthan a few microns.

Another common method of achieving layer transfer utilizes a hydrogenimplant followed by thermally induced layer splitting. Particles (atomsor ionized atoms, e.g., hydrogen atoms or a combination of hydrogen andhelium atoms) are implanted at a specified depth beneath the frontsurface of the donor wafer. The implanted particles form a cleave planein the donor wafer at the specified depth at which they were implanted.The surface of the donor wafer is cleaned to remove organic compounds orother contaminants, such as boron compounds, deposited on the waferduring the implantation process.

The front surface of the donor wafer is then bonded to a handle wafer toform a bonded wafer through a hydrophilic bonding process. Prior tobonding, the donor wafer and/or handle wafer are activated by exposingthe surfaces of the wafers to plasma containing, for example, oxygen ornitrogen. Exposure to the plasma modifies the structure of the surfacesin a process often referred to as surface activation, which activationprocess renders the surfaces of one or both of the donor water andhandle wafer hydrophilic. The surfaces of the wafers can be additionallychemically activated by a wet treatment, such as an SC1 clean orhydrofluoric acid. The wet treatment and the plasma activation may occurin either order, or the wafers may be subjected to only one treatment.The wafers are then pressed together, and a bond is formed therebetween. This bond is relatively weak, due to van der Waal's forces, andmust be strengthened before further processing can occur.

In some processes, the hydrophilic bond between the donor wafer andhandle wafer (i.e., a bonded wafer) is strengthened by heating orannealing the bonded wafer pair. In some processes, wafer bonding mayoccur at low temperatures, such as between approximately 300° C. and500° C. In some processes, wafer bonding may occur at high temperatures,such as between approximately 800° C. and 1100° C. The elevatedtemperatures cause the formation of covalent bonds between the adjoiningsurfaces of the donor wafer and the handle wafer, thus solidifying thebond between the donor wafer and the handle wafer. Concurrently with theheating or annealing of the bonded wafer, the particles earlierimplanted in the donor wafer weaken the cleave plane.

A portion of the donor wafer is then separated (i.e., cleaved) along thecleave plane from the bonded wafer to form the SOI wafer. Cleaving maybe carried out by placing the bonded wafer in a fixture in whichmechanical force is applied perpendicular to the opposing sides of thebonded wafer in order to pull a portion of the donor wafer apart fromthe bonded wafer. According to some methods, suction cups are utilizedto apply the mechanical force. The separation of the portion of thedonor wafer is initiated by applying a mechanical wedge at the edge ofthe bonded wafer at the cleave plane in order to initiate propagation ofa crack along the cleave plane. The mechanical force applied by thesuction cups then pulls the portion of the donor wafer from the bondedwafer, thus forming an SOI wafer. The donor can recycled for multipleuses as SOI donor wafer.

According to other methods, the bonded pair may instead be subjected toan elevated temperature over a period of time to separate the portion ofthe donor wafer from the bonded wafer. Exposure to the elevatedtemperature causes initiation and propagation of cracks along the cleaveplane, thus separating a portion of the donor wafer. The crack forms dueto the formation of voids from the implanted ions, which grow by Ostwaldripening. The voids are filled with hydrogen and helium. The voidsbecome platelets. The pressurized gases in the platelets propagatemicro-cavities and micro-cracks, which weaken the silicon on the implantplane. If the anneal is stopped at the proper time, the weakened bondedwafer may be cleaved by a mechanical process. However, if the thermaltreatment is continued for a longer duration and/or at a highertemperature, the micro-crack propagation reaches the level where allcracks merge along the cleave plane, thus separating a portion of thedonor wafer. This method allows recycle of the donor wafer, buttypically requires heating the implanted and bonded pair to temperaturesapproaching 500° C.

The use of high resistivity semiconductor-on-insulator (e.g.,silicon-on-insulator) wafers for RF related devices such as antennaswitches offers benefits over traditional substrates in terms of costand integration. To reduce parasitic power loss and minimize harmonicdistortion inherent when using conductive substrates for high frequencyapplications it is necessary, but not sufficient, to use substratewafers with a high resistivity. Accordingly, the resistivity of thehandle wafer for an RF device is generally greater than about 500Ohm-cm. As a handle wafer, wafers prepared according to the method ofthe present invention are particularly suitable for high resistivity SOIstructures for use in RF devices. An HR-SOI structure may comprise ahigh resistivity handle wafer having a low oxygen concentration, highresistivity, and be doped with germanium for improved mechanicalstrength. Accordingly, some embodiments of the present invention aredirected to an HR-SOI structure comprising a Ge-doped handle wafer, adielectric layer (generally, a buried silicon oxide layer or BOX), and adevice layer.

Such a substrate is prone to formation of high conductivity chargeinversion or accumulation layers at the BOX/handle interface causinggeneration of free carriers (electrons or holes), which reduce theeffective resistivity of the substrate and give rise to parasitic powerlosses and device nonlinearity when the devices are operated at RFfrequencies. These inversion/accumulation layers can be due to BOX fixedcharge, oxide trapped charge, interface trapped charge, and even DC biasapplied to the devices themselves.

A method is required therefore to trap the charge in any inducedinversion or accumulation layers so that the high resistivity of thesubstrate is maintained even in the very near surface region. It isknown that charge trapping layers (CTL) between the high resistivityhandle substrates and the buried oxide (BOX) may improve the performanceof RF devices fabricated using SOI wafers. A number of methods have beensuggested to form these high interface trap layers. For example, one ofthe method of creating a semiconductor-on-insulator (e.g., asilicon-on-insulator, or SOI) with a CTL for RF device applications isbased on depositing an undoped polycrystalline silicon film on a siliconsubstrate having high resistivity and then forming a stack of oxide andtop silicon layer on it. A polycrystalline silicon layer acts as a highdefectivity layer between the silicon substrate and the buried oxidelayer. An alternative method is the implantation of heavy ions to createa near surface damage layer. Devices, such as radiofrequency devices,are built in the top silicon layer.

It has been shown in academic studies that the polycrystalline siliconlayer in between of the oxide and substrate improves the deviceisolation, decreases transmission line losses and reduces harmonicdistortions. See, for example: H. S. Gamble, et al. “Low-loss CPW lineson surface stabilized high resistivity silicon,” Microwave Guided WaveLett., 9(10), pp. 395-397, 1999; D. Lederer, R. Lobet and J.-P. Raskin,“Enhanced high resistivity SOI wafers for RF applications,” IEEE Intl.SOI Conf., pp. 46-47, 2004; D. Lederer and J.-P. Raskin, “New substratepassivation method dedicated to high resistivity SOI wafer fabricationwith increased substrate resistivity,” IEEE Electron Device Letters,vol. 26, no. 11, pp. 805-807, 2005; D. Lederer, B. Aspar, C. Lagháe andJ.-P. Raskin, “Performance of RF passive structures and SOI MOSFETstransferred on a passivated HR SOI substrate,” IEEE International SOIConference, pp. 29-30, 2006; and Daniel C. Kerret al. “Identification ofRF harmonic distortion on Si substrates and its reduction using atrap-rich layer”, Silicon Monolithic Integrated Circuits in RF Systems,2008. SiRF 2008 (IEEE Topical Meeting), pp. 151-154, 2008.

In some embodiments, the high resistivity, low oxygen, germanium dopedwafers are suitable substrates for epitaxial deposition. The epitaxialdeposition preferably is carried out by chemical vapor deposition.Generally speaking, chemical vapor deposition involves exposing thesurface of the wafer to an atmosphere comprising silicon in an epitaxialdeposition reactor, e.g., a Centura reactor available from AppliedMaterials. Preferably, the surface of the wafer is exposed to anatmosphere comprising a volatile gas comprising silicon (e.g., SiCl₄,SiHCl₃, SiH₂Cl₂, SiH₃Cl, or SiH₄). The atmosphere also preferablycontains a carrier gas (preferably H₂). For example, the source ofsilicon during the epitaxial deposition may be SiH₂Cl₂ or SiH₄. IfSiH₂Cl₂ is used, the reactor vacuum pressure during depositionpreferably is from about 500 to about 760 Torr. If, on the other hand,SiH₄ is used, the reactor pressure preferably is about 100 Torr. Mostpreferably, the source of silicon during the deposition is SiHCl₃. Thistends to be much cheaper than other sources. In addition, an epitaxialdeposition using SiHCl₃ may be conducted at atmospheric pressure. Thisis advantageous because no vacuum pump is required and the reactorchamber does not have to be as robust to prevent collapse. Moreover,fewer safety hazards are presented and the chance of air or other gasesleaking into the reactor chamber is lessened.

During the epitaxial deposition, the temperature of the wafer surfacepreferably is ramped to and maintained at a temperature sufficient toprevent the atmosphere comprising silicon from depositingpolycrystalline silicon on the surface. Generally, the temperature ofthe surface during this period preferably is at least about 900° C. Morepreferably, the temperature of the surface is maintained in the range ofbetween about 1050 and about 1150° C. Most preferably, the temperatureof the surface is maintained at the silicon oxide removal temperature.

The rate of growth of the epitaxial deposition preferably is from about0.5 to about 7.0 μm/min. A rate of about 3.5 to about 4.0 μm/min may beachieved, for example, by using an atmosphere consisting essentially ofabout 2.5 mole % SiHCl₃ and about 97.5 mole % H₂ at a temperature ofabout 1150° C. and an absolute pressure of up to about 1 atm.

In some applications, the wafers comprise an epitaxial layer whichimparts electrical properties. In some embodiments, the epitaxial layeris lightly doped with phosphorous. Therefore, the ambient for epitaxialdeposition comprises phosphorous present as a volatile compound, suchas, for example, phosphine, PH₃. In some embodiments, the epitaxiallayer can contain boron. Such a layer may be prepared by, for example,including B₂H₆ in the atmosphere during the deposition.

A deposited epitaxial layer may comprise substantially the sameelectrical characteristics as the underlying wafer. Alternatively, theepitaxial layer may comprise different electrical characteristics as theunderlying wafer. An epitaxial layer may comprise a material selectedfrom the group consisting of silicon, silicon carbide, silicongermanium, gallium arsenide, gallium nitride, indium phosphide, indiumgallium arsenide, germanium, and combinations thereof. Depending uponthe desired properties of the final integrated circuit device, theepitaxial layer may comprise a dopant selected from the group consistingof boron, arsenic, and phosphorus. The resistivity of the epitaxiallayer may range from 1 to 50 Ohm-cm, typically, from 5 to 25 Ohm-cm. Insome embodiments, the epitaxial layer may have a thickness between about20 nanometers and about 3 micrometers, such as between about 20nanometers and about 2 micrometers, such as between about 20 nanometersand about 1.5 micrometers or between about 1.5 micrometers and about 3micrometers.

In some embodiments, the high resistivity, low oxygen, germanium dopedwafers are suitable substrates for gallium nitride epitaxial deposition,such as by molecular beam epitaxy. GaN molecular beam epitaxy (MBE)growth is a non-equilibrium process where a Ga vapor beam from aneffusion cell and an activated nitrogen beam from a plasma source aredirected toward a heated substrate. Under suitable conditions,layer-by-layer deposition of Ga and N atomic planes is possible. The MBEprocedure is performed in an ultra-high vacuum chamber, minimizing filmcontamination.

The following non-limiting Examples are provided to further illustratethe present invention.

EXAMPLES Example 1. Crystal Growth

A germanium doped single crystal silicon short ingot (crystal ID #1) anda germanium doped single crystal silicon full length ingot (crystal ID#2) were produced in a 200 mm FF furnace (SunEdison, S. Korea).Polycrystalline silicon and germanium were charged to a high purityquartz lined synthetic crucible (Toshiba). The charge for preparing thegermanium doped single crystal silicon full length ingot (crystal ID #2)comprised 1.3 kg of 5N grade germanium and 180 kg of high resistivitypolycrystalline silicon (>1000 Ohm·Cm). Additionally, 0.024 grams ofphosphorus was added to the melt to provide a phosphorus dopantconcentration of about 1.1×10⁶ ppba. The charge was melted and a crystalwas pulled according to the techniques disclosed herein. The singlecrystal silicon ingot was targeted for >16,000 Ohm·Cm resistivity byadjusting the dopant concentration in the melt before starting thecrystal growth process. This was based on resistivity of the meltcalculated by growing a short crystal with <200 mm diameter and <15 kgwt. The single crystal silicon ingot comprises <5.0 ppma Oi.

Example 2. Resistivity of Annealed Ingot

The germanium doped single crystal silicon full length ingot (crystal ID#2) prepared according to Example 1 was subjected to a thermal annealprocedure. Prior to anneal the pulled ingot, the pulled ingot wascropped by removing the seed and end cones. For analysis, the ingot wassliced at different positions, thereby preparing multiple slugs having athickness of about 1350 micrometers. Each cropped slug was edge groundand subjected to a mixed acid etch to a final thickness of about 1180micrometers, which was followed by lapped wafer cleaning to a finalthickness of 1150 micrometers and pre-RTA clean. The slug was subjectedto a thermal donor kill in a rapid thermal anneal at 750° C. at atemperature ramp of 360° C./minute. After lapping and anneal, theannealed slugs were cooled and held for four hours before four pointprobe measurement. The annealed slug was subjected to a four point probemeasurement technique for resistivity and additional characteristics, asprovided in Table 2.

TABLE 2 Resistivity Resistivity after thermal after donor donor killgeneration Recharge Position Oi anneal anneal Ingot ID Sequence (mm)(PPMA) (ohm cm) (ohm cm) #1 (Short First 0 12.942 Piece) 150 2.879 #2(Last, First 150 3.225 24556 20560 180 kg) 445 2.564 51819 83720 6873.211 224800 354200

Example 3. Wafer Mechanical Strength

The mechanical strength of a single crystal silicon wafer sliced fromthe ingot prepared according to Example 1 is compared withnon-germanium-doped silicon wafer (ID #0) also having low Oi using anEPI reactor slip generation test by a temperature ramp. According tothis test, the lower the number of slip generated, the greater themechanical strength. Thereby, a lower yield loss is expected. Further,the larger the no-slip temperature window by this test, the larger theprocess window during the SOI wafer and device manufacturing. Asdepicted by the graphs of FIGS. 10A and 10B, germanium doping andnitrogen doping provide a significant improvement of slip count. Thenon-germanium-doped silicon wafer demonstrated a ˜3° C. temperatureoffset window at 1100° C., 250 s process condition. In comparison, thegermanium doped wafer demonstrated a ˜6° C. temperature offset windowunder similar condition. This improvement is significant enough toimprove slip free processing of high resistivity low Oi wafer under muchsevere process condition.

Example 4. Wafer Mechanical Strength

The mechanical strength of a single crystal silicon wafer sliced fromthe ingot prepared according to Example 1 is compared withnon-germanium-doped silicon wafer (ID #0) also having low Oi using anEPI reactor slip generation test by a temperature ramp. According tothis test, the lower the number of slip generated, the greater themechanical strength. Thereby, a lower yield loss is expected. Further,the larger the no-slip temperature window by this test, the larger theprocess window during the SOI wafer and device manufacturing. Asdepicted by the graph of FIG. 11 , germanium doping provides asignificant improvement of slip count. The non-germanium-doped siliconwafer demonstrated a ˜3° C. temperature offset window at 1100° C., 250sprocess condition. In comparison, the germanium doped wafer demonstrateda ˜6° C. temperature offset window under similar condition. Thisimprovement is significant enough to improve slip free processing ofhigh resistivity low Oi wafer under much severe process condition.

Example 5. Mechanical Strength

A nitrogen doped single crystal silicon ingot was produced in 200 mm FFfurnace at SunEdison Semiconductor facility at S. Korea. Seed endN-concentration was targeted for ˜1.4×10¹⁴ nitrogen atoms/cm³ in thewafer by adding equivalent amount of silicon nitride during meltdownprocess (crystal ID #3). Similarly, another crystal was grown bytargeting ˜5×10¹⁴ nitrogen atoms/cm³ concentration at seed end (crystalID #4). Resistivity of the crystal was targeted for >3000 Ohm·Cm at seedend by adjusting the dopant concentration in the melt before startingthe crystal growth process (based on resistivity of the melt calculatedby growing a short crystal with <200 mm diameter and <15 kg wt). Theprocess is optimized to get desired <6.0 ppma Oi.

A germanium doped single crystal silicon ingot (crystal ID #2) wasproduced in a 200 mm FF furnace at SunEdison facility at S. Korea. 1.3kg on 5N grade Ge is co-melted with 180 kg of high resistivity Si (>1000Ohm·Cm) in a high purity quartz lined synthetic crucible. Crystal wastargeted for >16,000 Ohm·Cm resistivity by adjusting the dopantconcentration in the melt.

The mechanical strength of the wafers are compared withnon-germanium-doped silicon wafer (high resistivity, >1000 Ohm·Cm; andlow Oi, <6.0 ppma) and different concentration of nitrogen and Ge dopedlow Oi using an EPI reactor slip generation test by a temperature ramp.The lower the number of slip generated by this test, the greater themechanical strength, and lower yield loss is expected. Further, largerthe no-slip temperature window, larger the process window during SOIwafer and device manufacturing. Based on this test, these is asignificant improvement of slip count is observed. Thenon-germanium-doped silicon wafer has ˜3° C. temperature offset windowat 1100° C., 250s process condition, while the low concentrationnitrogen doping (˜1.4×10¹⁴ nitrogen atoms/cm³) has 9° C. and highconcentration N doping (˜5×10¹⁴ nitrogen atoms/cm³) has >20° C.temperature offset. See Table 3. Ge doped wafer has ˜6° C. temperatureoffset window under similar condition. This improvement is significantenough to improve slip free processing of high resistivity low Oi waferunder much severe process condition.

TABLE 3 Slip Window Interstitial at 1100° C., Ingot ID Dopant and Oxygen250 s Concentration Concentration (PPMA) (° C.) #0 Non-doped 2.58 3 #27.00 × 10¹⁹ germanium 3.45 6 atoms/cm³ #3 2.05 × 10¹⁴ nitrogen 2.9 9atoms/cm³ #4 7.50 × 10¹⁴ nitrogen 3.7 20 atoms/cm³

When introducing elements of the present invention or the embodiment(s)thereof, the articles “a”, “an”, “the” and “said” are intended to meanthat there are one or more of the elements. The terms “comprising”,“including” and “having” are intended to be inclusive and mean thatthere may be additional elements other than the listed elements.

Approximating language, as used herein throughout the specification andclaims, may be applied to modify any quantitative representation thatcould permissibly vary without resulting in a change in the basicfunction to which it is related. Accordingly, a value modified by a termor terms, such as “about,” “approximately,” and “substantially,” is notto be limited to the precise value specified. In at least someinstances, the approximating language may correspond to the precision ofan instrument for measuring the value. Here and throughout thespecification and claims, range limitations may be combined and/orinterchanged; such ranges are identified and include all the sub-rangescontained therein unless context or language indicates otherwise.

As various changes could be made in the above without departing from thescope of the invention, it is intended that all matter contained in theabove description and shown in the accompanying drawings shall beinterpreted as illustrative and not in a limiting sense.

What is claimed is:
 1. A single crystal silicon wafer comprising: twomajor, parallel surfaces, one of which is a front surface of the singlecrystal silicon wafer and the other of which is a back surface of thesingle crystal silicon wafer, a circumferential edge joining the frontand back surfaces of the single crystal silicon wafer, a bulk regionbetween the front and back surfaces, and a central plane of the singlecrystal silicon wafer between the front and back surfaces of the singlecrystal silicon wafer, wherein: (a) the bulk region comprises nitrogenat a nitrogen concentration of at least about 1×10¹⁴ atoms/cm³; (b) thebulk region comprises germanium at a germanium concentration of at leastabout 1×10¹⁹ atoms/cm³ and less than about 1×10²² atoms/cm³; (c) thebulk region comprises interstitial oxygen at an interstitial oxygenconcentration of less than about 6 ppma (New ASTM: ASTM F 121,1980-1983; DIN 50438/1, 1978); and (d) the bulk region of the singlecrystal silicon wafer has a resistivity of at least about 10,000 ohm cm.2. The single crystal silicon wafer of claim 1 wherein the interstitialoxygen concentration is less than about 5 ppma.
 3. The single crystalsilicon wafer of claim 1 wherein the interstitial oxygen concentrationis less than about 4 ppma.
 4. The single crystal silicon wafer of claim1 wherein the interstitial oxygen concentration is less than about 3ppma.
 5. The single crystal silicon wafer of claim 1 wherein theresistivity is at least about 15,000 ohm cm.
 6. The single crystalsilicon wafer of claim 1 wherein the resistivity is at least about20,000 ohm cm.
 7. The single crystal silicon wafer of claim 1 whereinthe nitrogen concentration is at least about 5×10¹⁴ atoms/cm³ and lessthan about 1×10¹⁶ atoms/cm³.
 8. The single crystal silicon wafer ofclaim 1 wherein the nitrogen concentration is at least about 1×10¹⁵atoms/cm³ and less than about 1×10¹⁶ atoms/cm³.
 9. The single crystalsilicon wafer of claim 1 further comprising germanium at a germaniumconcentration of at least about 5×10¹⁹ atoms/cm³ and less than about1×10²² atoms/cm³.
 10. A method of growing a single crystal siliconingot, the method comprising: preparing a silicon melt, wherein thesilicon melt is prepared by melting polycrystalline silicon in a quartzlined crucible and adding a source of nitrogen to the quartz linedcrucible and adding a source of germanium to the quartz lined crucible;and pulling the single crystal silicon ingot from the silicon melt, thesingle crystal silicon ingot comprising a central axis, a crown, an endopposite the crown, and a main body between the crown and the oppositeend, the main body having a lateral surface and a radius, R, extendingfrom the central axis to the lateral surface, wherein the main body ofthe single crystal silicon ingot comprises nitrogen at a nitrogenconcentration of at least about 1×10¹⁴ atoms/cm³, wherein the main bodyof the single crystal silicon ingot further comprises germanium at agermanium concentration of at least about 1×10¹⁹ atoms/cm³ and less thanabout 1×10²² atoms/cm³, wherein the pulling conditions are sufficient toyield an interstitial oxygen concentration in the main body of thesingle crystal silicon ingot of less than about 6 ppma (New ASTM: ASTM F121, 1980-1983; DIN 50438/1, 1978), and further wherein the main body ofthe single crystal silicon ingot has a resistivity of at least about10,000 ohm cm.
 11. The method of claim 10 wherein the pulling conditionsare sufficient to yield an interstitial oxygen concentration in the mainbody of the single crystal silicon ingot of less than about 5 ppma. 12.The method of claim 10 wherein the pulling conditions are sufficient toyield an interstitial oxygen concentration in the main body of thesingle crystal silicon ingot of less than about 4 ppma.
 13. The methodof claim 10 wherein the pulling conditions are sufficient to yield aninterstitial oxygen concentration in the main body of the single crystalsilicon ingot of less than about 3 ppma.
 14. The method of claim 10wherein the main body of the single crystal silicon ingot has aresistivity of at least about 15,000 ohm cm.
 15. The method of claim 10wherein the main body of the single crystal silicon ingot has aresistivity of at least about 20,000 ohm cm.
 16. The method of claim 10wherein the main body of the single crystal silicon ingot comprisesnitrogen at a nitrogen concentration of at least about 5×10¹⁴ atoms/cm³and less than about 1×10¹⁶ atoms/cm³.
 17. The method of claim 10 whereinthe main body of the single crystal silicon ingot comprises nitrogen ata nitrogen concentration of at least about 1×10¹⁵ atoms/cm³ and lessthan about 1×10¹⁶ atoms/cm³.
 18. The method of claim 10 furthercomprising adding a source of germanium to the quartz lined crucible andwherein the main body of the single crystal silicon ingot furthercomprises germanium at a germanium concentration of at least about5×10¹⁹ atoms/cm³ and less than about 1×10²² atoms/cm³.
 19. A singlecrystal silicon ingot comprising: a central axis, a crown, an endopposite the crown, and a main body between the crown and the oppositeend, the main body having a lateral surface and a radius, R, extendingfrom the central axis to the lateral surface, wherein: (a) the main bodyof the single crystal silicon ingot comprises nitrogen at a nitrogenconcentration of at least about 1×10¹⁴ atoms/cm³; (b) the main body ofthe single crystal silicon ingot comprises germanium at a germaniumconcentration of at least about 1×10¹⁹ atoms/cm³ and less than about1×10²² atoms/cm³; (c) the main body of the single crystal silicon ingotcomprises interstitial oxygen at an interstitial oxygen concentration ofless than about 6 ppma (New ASTM: ASTM F 121, 1980-1983; DIN 50438/1,1978); and (d) the main body of the single crystal silicon ingot has aresistivity of at least about 10,000 ohm cm.
 20. The method of claim 19wherein the interstitial oxygen concentration is less than about 5 ppma.21. The method of claim 19 wherein the interstitial oxygen concentrationis less than about 4 ppma.
 22. The method of claim 19 wherein theinterstitial oxygen concentration is less than about 3 ppma.
 23. Themethod of claim 19 wherein the resistivity is at least about 15,000 ohmcm.
 24. The method of claim 19 wherein the resistivity is at least about20,000 ohm cm.
 25. The method of claim 19 wherein the nitrogenconcentration is at least about 5×10¹⁴ atoms/cm³ and less than about1×10¹⁶ atoms/cm³.
 26. The method of claim 19 wherein the nitrogenconcentration is at least about 1×10¹⁵ atoms/cm³ and less than about1×10¹⁶ atoms/cm³.
 27. The method of claim 19 further comprisinggermanium at germanium concentration of at least about 5×10¹⁹ atoms/cm³and less than about 1×10²² atoms/cm³.